A digital subscriber line (xDSL) is a high-speed data transmission technique on an unshielded twisted pair (UTP). FIG. 1 shows a system reference model of a system DSL access multiplexer DSLAM (DSL Access Multiplexer) that provides multiple xDSL access.
Due to electromagnetic induction, mutual interference occurs between multiple signals of DSLAM access, which is called crosstalk. As shown in FIG. 2, near-end crosstalk NEXT (Near-End Crosstalk) energy and far-end crosstalk FEXT (Far-End Crosstalk) energy increase with the increase of the frequency. Frequency division multiplexing is applied to xDSL uplink and downlink channels, and near-end crosstalk does not cause too much harm to system performance. However, when wider and wider bands are applied to xDSL, far-end crosstalk affects transmission performance of the line more seriously.
Currently, a vectored-DSL (vectored-DSL) technology is put forward in the industry. It primarily takes advantage of the possibility of joint transmitting and receiving on a DSLAM side and uses a signal processing method to cancel FEXT interference and finally eliminate FEXT interference in each signal. FIG. 3 and FIG. 4 illustrate working scenarios of synchronous transmitting and synchronous receiving on the DSLAM side respectively.
In vectored-DSL, a downlink precoding matrix P and an uplink cancellation matrix W need to be estimated. In a vectored-DSL system, the following steps are performed:
1. Sync symbol (Sync Symbol) alignment is implemented, where the sync symbol is a DMT symbol synchronization signal that carries a synchronization frame.
2. A vectoring control entity VCE (Vectoring Control Entity) allocates pilot sequences to all lines in a unified manner and an ONU-side VDSL2 transceiver unit at the Optical network unit (VTU-O) of each line jointly modulates the pilot sequence, which are allocated by the VCE, on the Sync Symbols of all lines.
3. The receiver side feeds back an error to the VCE.
The downlink precoding matrix P and the uplink cancellation matrix W can be estimated in the VCE first, and then the vectoring technology is applied to cancel the FEXT. The downlink precoding matrix is also known as a downlink crosstalk cancellation matrix or a downlink far-end crosstalk coefficient, and the uplink cancellation matrix is also known as an uplink crosstalk cancellation matrix or an uplink far-end crosstalk coefficient.
A process of initializing a new line that joins in (Join in) ordinary non-vectored VDSL2 lines or vectored-DSL vector lines in the prior art includes handshake (Handshake), channel discovery (Channel Discovery), training (Training), and channel analysis and exchange (Channel Analysis and Exchange). After initializing is finished, a data transmission (Showtime) stage comes. The Channel Discovery stage of an ordinary non-vectored VDSL2 line further includes an O-P-Channel-Discovery 1 stage and an R-P-Channel-Discovery 1 stage; and the Training stage further includes an O-P-Training 1 stage and an R-P-Training 1 stage. For the initializing of the vectored-DSL vector line, before the data transmission (Showtime) stage, an O-P-VECTOR 1 stage and an R-P-VECTOR 1 stage are inserted in the Channel Discovery stage, and an O-P-VECTOR 1-1 stage, an O-P-VECTOR 2-1 stage, an R-P-VECTOR 1-1 stage, an R-P-VECTOR 1-2 stage, and an R-P-VECTOR 2 stage are inserted in the Training stage. Within such stages, all and/or part of the downlink precoding matrix P and uplink cancellation matrix W may be estimated.
The vectored-DSL is a very-high-speed digital subscriber line 2 (VDSL2) for far-end self-crosstalk elimination. Because the VDSL2 technology is earlier than the vectored-DSL technology and has been applied widely, the upgrade from the VDSL2 to the vectored-DSL must allow for support of legacy lines, that is, ordinary non-vectored VDSL2 lines. A customer premises equipment (CPE) of a legacy line is a VDSL2 legacy CPE that does not support the vectored DSL. However, the VDSL2 legacy CPE does not support sending and receiving of a pilot sequence and feedback of an error on the Sync Symbol, which makes it difficult for the VCE to estimate the uplink and downlink far-end crosstalk coefficients intended for cancelling the crosstalk caused by the legacy line onto the vector line. If some lines in the system are in the data transmission (Showtime) stage, when a legacy line joins in the system, in the case that the crosstalk from the legacy line is not cancelled, the bit errors of the vector line in the Showtime stage will increase due to a lower signal-to-noise ratio (SNR), or even the vector line in the Showtime stage is deactivated and retrained. As a latent uncertain factor, the legacy line seriously affects the rate of the vector line and the stability of the entire vectored-DSL system.
If all VDSL2 legacy CPEs in the VDSL2 of the live network are upgraded to or replaced with vectored-DSL enabled vector customer premises equipment VDSL2 vector CPE, huge costs are required. Some old legacy CPEs may not be upgradable to the vector CPE for various reasons such as no support of error calculation, error feedback, or uplink sending of pilot sequences, which makes it necessary to replace the entire CPE and further increases costs.
As regards the issue of the vectored-DSL being down-compatible with the legacy CPE, a vector friendly (vector Friendly) CPE solution is put forward in the industry. Specifically, the solution specifies that the vector Friendly CPE must be able to identify and receive a pilot signal modulated on a downlink Sync Symbol, and additionally, a VTU-O controls downlink Sync Symbol alignment of all lines. When the vectored-DSL system meets the above two conditions, for the vector line, the VCE can estimate a downlink cancellation coefficient for cancelling legacy line crosstalk. In this way, the potential stability trouble caused by the legacy line onto the vector line in the entire vectored-DSL system is eliminated in the downlink direction. In the application of the vector Friendly solution, the legacy CPE in the live network still needs to be upgraded to the vector Friendly CPE, which requires a high cost. Because the vector Friendly CPE is unable to send uplink pilot signals, the VCE can hardly estimate the far-end crosstalk coefficient intended for cancelling the crosstalk caused by the legacy line in the uplink direction onto the vector line. Consequently, the potential stability trouble caused by the legacy line onto the vector line in the vectored-DSL system is not eliminated in the uplink direction.